Recently, there has come to be noted an optical information recording and reproducing apparatus whereby information can be recorded at a high density in an optical recording medium by collecting light beams and projecting them onto this recording medium and the recorded information written into the recording medium can be read out (reproduced) at a high speed by receiving with a light detector the light returning from this recording medium.
The above mentioned recording medium may be a photomagnetic disc wherein, on a substrate such as acrylate resin as PMMA, there is formed a magnetic recording layer on which the light beams having passed through this substrate are collected and projected and the polarized plane of the returning light rotates in response to the magnetized direction of the part forming a recording film or recording layer different in the returning light amount.
As disclosed in Japanese Patent Laid Open No. 74701/1982, the above mentioned acrylate resin is high in the optical characteristics but has a defect in that the hygroscopicity is so high as to deflect the recording medium surface.
Therefore, it is considered to be effective to use for the substrate a polycarbonate (abbreviated as PC hereinafter) resin or the like which is hard to deflect, high in form stability and high in mechanical strength.
In case the above mentioned PC resin or the like is to be used for the substrate, it will be necessary to well match its optical characteristics. For example, in case its refractive index is large, the optical distance per unit length will become so long that the thickness of the substrate will not be able to be made large. Therefore, it is necessary to investigate the refractive index of the material to be used for the substrate and, as the refractive index varies depending on the method of molding the substrate in some cases, it is desirable to measure the refractive index on the substrate form made by using an actual molding method.
FIG. 1 shows an apparatus 1 for measuring the double refractive index of a disc-shaped recording medium substrate which is a prior art example.
That is to say, a random polarized laser beam of an He-Ne laser 2 has its transmitted light beam made a predetermined linear polarized light beam by a polarizer 3 such as a Glan-Thompson prism (abbreviated as GTP) and is then projected onto a substrate 4 as a measured medium. The light bundle having passed through this substrate 4 passes through a phase compensating plate 5 of Babinet-Soleil arranged so as to be opposed to the above mentioned polarizer 3, then passes through a light analyzer 6 such as a GTP set in a light erasing position (crossed Nichol) so as to pass the polarized light intersecting at right angles with the above mentioned polarizer 3 and is received by a light receiving element 17. In the case of the polarized beam made incident upon the above mentioned substrate 4, if the substrate 4 is of a uniaxial (crystal) characteristic in which the optical axis becomes vertical to the substrate plane, even if the polarized direction is varied by rotating the polarizer 3, no phase difference (becoming elliptic) will be produced in the substrate 4. However, when the optical axis is in the substrate plane, a phase difference will be produced in response to the angle made by the polarized direction and the optical axis and, even in the case of a biaxial (crystal) characteristic, a phase difference will be produced in the substrate 4 by changing the polarized direction of the polarized beam. Therefore, if the phase difference produced in this substrate 4 is erased by moving in the vertical direction (in the paper plane) two wedges, for example, the right optical rotation plate 5b, among the left optical rotation plate 5a and right optical rotation plate 5b, in the phase compensating plate 5, and uniformly varying the thickness of the two plates, the light passing through the light analyzer 6 in the crossed Nichol state will be erased for the polarizer 3 and the signal output from the light receiving element 7 will become a minimum. The double refractive index in the plane of the substrate 4 can be measured from the displacement of the above mentioned phase compensating plate 5.
The refractive index in the thickness direction of the substrate 4 is not known at all in the measuring method of the above mentioned prior art. Therefore, this method is insufficient to be used for the substrate of an optical recording medium. That is to say, in the case of being projected onto the recording layer through the substrate of the recording medium, parallel light bundles are focused to be in the form of a spot and this light collecting angle or the number of apertures N.A. is considerably large. In the position of the substrate surface, the light bundles are kept defocused so as to be hard to be influenced by dust or the like. When the light bundles are thus collected, in case the substrate is of an optical material showing a double refractive index, the refractive index component in the thickness direction will influence the light bundles passing through the substrate. This fact shall be explained in the following.
In case an injection-molded PC plate is used for the above mentioned substrate, this substrate will show double refraction as in a uniaxial crystal and will have an optical axis in the direction vertical to the substrate plane in most cases. The refractive index (no) for ordinary light and refractive index (ne) for extraordinary light are different from each other.
Therefore, a linear polarized light incident upon this substrate as inclined with respect to the optical axis (in the direction vertical to the substrate plane) will produce a phase difference due to the double refraction when the angle formed by the polarizing direction and the plane of incidence is other than a specific angle and will produce an ellipse (the linear polarized light will become an elliptic polarized light).
The reason why such an ellipse is produced shall be explained in the following with reference to FIGS. 2 and 3.
FIG. 2 is an explanatory view showing how a laser beam 14 is pressed into a part of a substrate 12, forming a disc 11 of an objective, to irradiate it in the form of a spot. In the drawing, only a part of the disc 11 is shown.
The laser beam 14 is a linearly polarized light in which the polarized direction is a linearly polarized light intersecting at right angles with the radial direction 16 of the substrate 12 as shown by the reference numeral 15 and includes a beam portion (S polarized light) 21 incident as intersected at right angles with the polarized direction and a beam portion (P polarized light) 22 incident parallelly with the polarized direction, for example, beam portions 23 and 24 incident as inclined by 45 degrees respectively with respect to these beam portions. These beam portions 23 and 24 become polarized light including both components of the S polarized light and P polarized light.
On the other hand, the refractive indices for the S polarized light and P polarized light incident upon this substrate 12 as inclined by an angle .theta.i with respect to the optical axis (in the direction vertical to the substrate plane) indicated by the reference numeral 12a in FIG. 3 are determined as follows.
FIG. 3 is an explanatory view showing the relation between an incident angle .theta.i of the light incident upon the substrate 12 and the refractive index.
The injection-molded PC substrate shows a substantially uniaxial crystal characteristic and two of the main refractive indices n.sub.1, n.sub.2 and n.sub.3 are equal to each other. If a refractive index ellipsoid is indicated by selecting the axes of coordinates so that n.sub.1 =n.sub.2 and the Z axis direction may be n.sub.3, the optical axis 12a will coincide with Z axis.
Here, the refractive index n' for the S polarized light incident as inclined by the angle .theta.i with respect to the optical axis (the direction vertical to the substrate plane) 12a and the refractive index n" for the P polarized light are represented by the minor axis 26a and major axis 26b of the vertically sectioned area (ellipse 26) of the light 25' after the incidence. That is to say, if the angle formed by the light 25 after the incidence with the optical axis is .theta.t, EQU n'=n.sub.1 ( 1) EQU n"=n.sub.1 n.sub.3 /.sqroot.n.sub.1.sup.2 sin .sup.2 .theta.t+n.sub.3.sup.2 cos .sup.2 .theta.t (2)
Here, sin .theta.t=(1/n') sin .theta.i
Therefore, the beam portion 21 of the S polarized light incident upon the substrate 12 and the beam portion 22 of the P polarized light hold linear polarized lights but, for example, as the beam portions 23 and 24 incident as inclined by 45 degrees with respect to the above mentioned beam portions 21 and 22 are polarized lights including both components of the S polarized light and P polarized light a phase difference will be produced between the S polarized light component and the P polarized light component and the linearly polarized light will become an elliptically polarized light. If the thickness of the substrate 12 is represented by d and the wave length is represented by .lambda., this phase difference will be represented by EQU .delta..sub.s-p =(2.pi./.lambda.).multidot.(n'-n").multidot.(d/ cos .theta.t)
Therefore, the larger the thickness d of the substrate and the incident angle .theta.i, the larger the phase difference .delta..sub.s-p.
FIG. 4 is a cross-sectioned view of a beam incident upon the objective by a linear polarization of a polarized direction represented by the reference numeral 27, reflected by the disc 11 and then again passing through the objective. In the beam incident upon the substrate, the nearer to the peripheral edge side, that is, the larger the aperture, the larger the angle .theta..sub.1 of incidence and the phase difference .delta..sub.s-p. Where the orientation angle (the angle formed by the incident plane and polarizing direction) corresponds to 45 degrees (that is 45 and 135 degrees), that is, at the reference numerals 28a, 28b, 28c and 28d, The ellipticity will become maximum.
Thus, in case the substrate shows a double refraction, even if the double refraction is of a uniaxial characteristic, by the refractive index for the thickness direction, the linear polarized light will become an elliptic light having a polarized light component at right angles with the linear polarized light direction.
Therefore, for example, in the case of being used for a substrate of a photomagnetic disc, the polarized direction of a returning light, in case a linearly polarized light is radiated, will rotate by a minute angle in response to the magnetizing direction but, even if a light analyzer is set so as to pass only the rotated polarized light component, the light beam having passed through the substrate will be made elliptic and therefore the beams other than the inherent signal component will also pass through this light analyzer and will mix into the signal. Also, there will be produced a signal component intercepted by the analyzer due to the ellipticity. Thus, the C/N (carrier to noise ratio) will reduce.
The above mentioned ellipticity will be produced by the difference between n' and n". This refractive index n" derives from the the refractive index in the thickness direction.
Thus, in the case of being used for the substrate of a photomagnetic disc, the refractive index in the thickness direction will become a very important factor but its value can not be determined in the above mentioned prior art example.
Also, in the case of being used not only for the substrate of a photomagnetic disk but also for the substrate of a photodisc for the reproduction or the like of recorded information by the difference of the reflected light amount, there is extensively used an optical system wherein a light beam, made of circular polarized light from a linear polarized light having passed through a polarized beam splitter by using a .lambda./4 plate, is projected and this returning light is again made a linear polarized light in a polarized direction intersecting at right angles with the above mentioned linear polarized direction by the .lambda./4 and is efficiently branched to the information light detector side by the above mentioned beam splitter. However, in this case, too, due to the refractive index in the thickness direction of the substrate (having a value different from the refractive index in the substrate plane direction), the light will not be efficiently branched and the C/N (carrier to noise) reduced. Also, in case the lights are collected and projected, if the refractive index in the thickness direction of the substrate is different from the refractive index in the substrate plane, the light beam will not be sufficiently focused, the beam spot will become larger than in the case of the isotropic refractive index and therefore it will be unfavorable in the case of high density recording. In the case of the recording mode, the energy density will reduce, therefore the output of the light source will have to be made larger and high speed recording will be obstructed.
That is to say, in the case of not only a photomagnetic disc but also a photodisc, it is very important to know the value of the refractive index in the thickness direction of the substrate but, with the measuring apparatus of the above mentioned prior art example, the refractive index in the thickness direction can not be determined.